Root access losted after installing stock/custom rom ? Or by flashing the old backup???OK OK I know that because that's why you are here
Don't worry you don't have to reroot it again :thumbup:
Just do what I said below

Code:

1 . I think the custom recovery will there (may be the cwm or twrp )
2 . Download the RootGainer zip from here .
3 . Flash it using the recovery and reboot your device

Now you are going to amaze Note :- This zip will only replaces the su values ... Flashing this zip may or may not be cause brick . I am not gonna take any responsibilities . Just think what you are doing before flashing.
It works on my Galaxy S Duos and maybe work for yours (my processor is arm )

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